The present invention relates to a solid state laser apparatus having a slab geometry laser medium whose cross section is rectangular, particularly relates to reduction of thermal lensing in the laser medium.
FIG. 1 is a cross sectional view of the structure of a conventional slab geometry solid state laser apparatus which is disclosed in a magazine (Laser Focus/E-0 TECHNOLOGY, SEPTEMBER, 1983 P. 106). In the figure, numeral 1 designates a laser medium; 1a, optically flat surfaces of the laser medium 1; 1b, optically non-smooth surfaces which intersect the optical flat surfaces 1a along the optical axis; 2, thermal insulators bonded to each of the non-smooth surfaces 1b; 5, a flow path of a coolant 4 for cooling the laser medium 1; 6, a circulating direction of the coolant 4; 7, a pumping lamp; 8, a pair of reflecting mirrors; and 71, pumping light.
By referring to FIG. 1, the operation of the apparatus will be described in the following.
In FIG. 1, the pumping light 71 radiated from the pumping lamp 7 is reflected by the reflecting mirrors 8 and then absorbed by the laser medium 1. Part of the energy is extracted by a pair of resonance mirrors (not shown in the figure) to the outside of the laser medium as a laser beam. The absorbed energy which is not used for the laser oscillation is converted into a thermal energy in the laser medium 1. The thermal energy heats up the laser medium 1. The laser medium 1 is cooled by the coolant 4 which is circulated in the flow path 5 so as to keep it at a predetermined temperature.
FIG. 2 is a view showing a heat flow, a temperature distribution, and thermal lens distribution generated by a temperature distribtion of the aforementioned laser medium 1.
If the heat generation in the laser medium 1 is uniform, the cooling effect on the optically flat surfaces 1a is uniform, and the heat insulation of the non-smooth surfaces 1b is perfect, then the temperature distribution in the width direction of the laser medium becomes uniform and thereby a thermal lens does not take place.
Even if the heat generation and cooling of the laser medium are uniform, the perfect insulation of the heat flow from the non-smooth surfaces 1b is impossible. When the temperature of the thermal insulators 2 becomes very high, much heat flows from the thermal insulators 2 to the laser medium 1 and then a temperature distribution is formed in the width direction A of the laser medium 1.
Actually, the thermal insulators 2 absorb the pumping light 71 from the pumping lamp 7 and thereby the temperature of the thermal insulators 2 becomes very high as shown by curve B of FIG. 2. Although the members 2 bonded to the side surfaces 1b are the thermal insulators, much heat flows to the laser medium 1 as shown by arrow 9 of FIG. 2. Thus, a temperature distribution as shown by curve B of FIG. 2 is formed. In the figure, To represents a water temperature. Consequently, a thermal lens is formed as shown by curve C of FIG. 2. In the figure, f is a focal length. It is clearly understood that a negative lens is formed.
Further, as shown in FIG. 3, even if the laser medium 1 is uniformly irradiated with the pumping light, strong heat generation may occur in bonding layers 30 with which the thermal insulators 2 are bonded to the laser medium 1. Since the thermal conductivity of the laser medium 1 is large enough compared with that of the thermal insulator, the heat generated in the bonding layers 30 is removed by the laser medium as shown by arrow 9 of FIG. 3. Thus, as shown by curve B of FIG. 3, a temperature distribution takes place in the width direction A of the laser medium 1, and a thermal lens takes place as shown by curve C of FIG. 3.
As was described above, in the conventional slab geometry solid state laser apparatus, a temperature distribution takes place in the width direction of the laser medium 1. Thus, a thermal lens occurs. The thermal lens results in distorting a laser beam pattern and decreasing the wall plug efficiency of the laser oscillation.